I assume he needed it to have a small opening in the cage to shove the Raspberry Pi through it (to broadcast new SSIDs)

The door of a microwave typically doesn't form an RF-tight seal. Instead there's a groove that forms a resonant trap at the microwave's operating frequency. So it'll probably block 2.4-GHz ISM-band stuff like Bluetooth (I don't actually know how wide the trap band is compared to a BT or wifi channel), but outside that band all bets are off.

You are replying to the article author. He knows you can use a microwave oven as a very cheap faraday cage. He tried that, but it wasn't good enough.